In cold climates the Cold Filter Plugging Point (CFPP) (EN116) of diesel fuels is very important and is specified in various standards such as the European diesel specification, EN590, where the climate related requirements vary from −20° C. CFPP for countries such as Germany to −10° C. and −5° C. respectively for countries such as Portugal and Greece. Countries such as Switzerland, Finland, Sweden, Norway and Denmark have adopted EN590 artic grade cold flow requirements with winter CFPP ranges being from −20° C. (Artic grade 0) to −32° C. (Arctic grade 2).
The cold flow behaviour of diesel fuels generally depends on their molecular structure. Fuels usually contain a mixture of hydrocarbons including n-paraffins, branched linear paraffins, olefins, aromatics and other non-polar and polar compounds. The straight chain hydrocarbons which have the lowest solubility in the fuel tend to separate as waxes at low temperatures below the cloud point of the fuel. The n-paraffins distribution of diesels is typically in the range of C9-C28 although the carbon chain length sometimes extends to the mid to upper thirties. As the chain length of the n-alkane molecule increases, its solubility in the fuel at low temperatures decreases and the rate of separation increases. Upon continuous lowering of temperature below the fuel cloud point, these waxes start to adhere together to form a network which eventually prevents the flow of the fuel as measured by the pour point test. Also the large wax platelets formed tend to block the diesel fuel filter and prevent the engine operation at temperatures below the fuel cloud point. This behaviour can be simulated using lab tests such as the cold filter plugging point (CFPP) test.
The addition of cold flow additives such as ethylene vinyl acetate (EVA) based co-polymers, tend to enhance the cold flow characteristics of these fuels. These additives function by reducing the size and changing the shape of the wax crystals. They also reduce the tendency of the crystals to adhere together and form a gel. Flow improvers are most effective in fuels with a low concentration of widely distributed waxy n-paraffins, since crystal growth is slow in such fuels and flow improver molecules can effectively co-crystallize on slowly growing wax crystals.
As a fuel is cooled to its cloud point, the normal paraffins begin to separate from the fuel wax. Upon further cooling, more wax appears and adds to these initial crystals. These crystals rapidly grow to a size which prevents fuel flow. Flow improvers act to modify the wax as it forms in the following ways:
Nucleation: Additive composition is adjusted such that at the fuel cloud point many artificial nuclei become available on which wax crystals grow.
Growth arresting: During crystal growth around the nuclei, additive molecules also act to prevent further growth.
Both of these effects combine and result in the formation of many very small crystals rather than fewer larger crystals. These small crystals pass through the filters and/or form permeable cakes on the filter medium to allow continued operability until the fuel has warmed and the wax redissolves.
It is believed that, amongst other factors, the following factors affect a fuel's response to flow improver additive:                Size of the crystal formed        The rate of wax precipitation        Wax carbon number range        Fractionation sharpness        Wax content and type.        
Narrow cut fuels, which are fractionated sharply, tend to be less responsive to flow improvers because they have a higher wax precipitation rate. It is generally agreed that flow improvers reduce filter plugging temperatures by co-crystallizing with n-paraffin molecules to inhibit wax crystal growth. This implies there is a balance between the rate of crystal growth and the rate of co-crystallization. If the rate of crystal growth is slow, the flow improver has a better chance of co-crystallization with the growing wax crystal and inhibits its growth. If the rate of crystal growth is rapid, large crystals form before the flow improvers can co-crystallize with them to hinder their growth. Fuels with a wide carbon distribution contain many different n-paraffinic molecules and it is believed that crystals from a mixture of n-paraffins grow at a slower rate than crystals formed from a single n-paraffin, because n-paraffins in mixtures do not line up side by side to form a new layer on the crystal. Since mixed n-paraffin crystals grow slowly, flow improvers have more time to interact with the growing crystals and inhibit their growth.
FT derived diesel consists of approximately 50% n-paraffins compared to an EN590 conventional diesel that contains less than 20% n-paraffins. Although FT derived diesel has a normal boiling range, comparable to that of EN590 diesels, the large total volume of n-paraffins may enhance crystal growth rate to the extent that it decrease the effectiveness of flow improvers compared to conventional diesels
It was thus expected that if FT derived diesel were blended with a crude oil derived diesel this would reduce the effectiveness of the CFPP additives on the blend.
Moreover, it was expected that a narrow cut FT derived diesel would reduce the effectiveness of the CFPP additives on the blend.
Surprisingly the inventors have solved the problem of the CFPP of FT derived diesel and crude oil derived diesel blends in the presence of CFPP additives.